Before leaving on my trip to the Trinity Site I built a simple hand-held computer of sorts to gather data from a Geiger counter and GPS receiver. The goal was to measure the radiation at Trinity Site and a couple of other places along the trip. I also wanted to see the effect of altitude on the background count.
The hardware used is shown at left. The computer is based on the Dallas Semiconductor DS-5000 microcontroller which is a clone of the Intel 8051 with an added boot loader in ROM and on-board nonvolatile SRAM. Power for the setup is supplied by either 6 "C" cells with a life of about 27 hours, an automobile cigarette lighter adapter or a 9V wall wart. Program loading and data retrieval is accomplished through an RS-232 port hooked to a laptop computer. All data is stored interally as well as being displayed on a 16x4 LCD module. The computer can run for about 13 hours before the 28K of available data memory is full.
The geiger counter is an Aware Electronics RM-60 Micro Roentgen Radiation Monitor. This unit has no readout of its own but interfaces to a computer serial or parallel port through a standard phone cable. An optional earphone "clicker" can be purchased to power the unit separately off a 9V battery and to listen to the radiation "clicks" directly. In this case the unit was attached to the microcontroller through unused ports of a MAX232 chip used to generate RS-232 voltage levels for the computer interface. The monitor holds the I/O line at a high level until an event occurs. It then pulls the I/O pin low for about 50 microseconds. The I/O line was constantly polled by the microcontroller so these events could be counted. Each 1.05 events/minute corresponds to 1.0 microRoentgen/hr of radiation. The device resolution is thus about 1 µR/hr.
The GPS receiver is a Garmin GPS 25LP OEM board mounted in a cheap aluminum case. This unit sends NMEA sentences through a standard RS-232 port which is wired in parallel with the above mentioned computer interface. The antenna is a standard Garmin GA-27. For the driving parts of the trip the antenna was stuck in the middle of the dashboard where it easily tracked enough satellites for good data. When walking, this mess was stuck in a camera bag for convenience with the antenna zipped inside. This caused no problems with satellite acquisition. Besides position and elevation information, the GPS receiver is used as the source for the time each data point is taken. Each minute it sends a sentence to the computer causing an interrupt. At this point, all data is stored to memory and updated on the display. The radiation count is then reset and the count resumes until the next sentence is received.
Below is a crude schematic and the code for the setup. The first code is the main routine for running this thing. The second is a separate routine that is loaded into memory to dump the contents.
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Latest update: August 18, 2005 |
